1. Field of the Invention
The invention relates generally to radiation gauges, including x-ray and isotope gauges, used for measuring the thickness of objects. More particularly, the invention relates to linearization systems and processes for use in radiation gauges which preserve the resolution inherent in the non-linear signal throughout the conversion process.
2. Brief Description of the Prior Art.
Radiation gauges are commonly used for determining the thickness of an object, such as a sheet of rolling steel, inserted between a radiation source and a detector located opposite the source. The radiation beam passed through the object is attenuated thereby and the residual beam intensity may be used by the detector to develop a non-linear analog signal representative of the thickness of the object. The non-linear signal is developed in accordance with Bouquer's exponential law of absorption I = I.sub.o e.sup.-.mu.x where; I is the final (residual) intensity of the radiation detected after the beam has passed through an object the thickness of which is being measured; I.sub.o is the initial intensity of the beam; .mu. is the transmission (permeability) coefficient of the object; and x (to be determined) is the thickness of the object.
Photomultiplier tube detectors are commonly used to perform the actual conversion of the residual beam into a non-linear analog signal (a voltage) V.sub.I which varies in accordance with Bouquer's law.
In order to drive a variety of output devices which respond only to a linear signal the non-linear signal developed by the detector, V.sub.I, must be converted to linear form. Furthermore, the conversion must result in the development of a high resolution signal in order to accurately indicate the thickness of objects over the entire range of gauge operation.
For the sake of illustration, assume that it is desirable for a gauge to operate within a range of zero volts to 10 volts, i.e., 0.ltoreq.V.sub.I .ltoreq.10. This is a realistic range of gauge operation and may therefore be used to illustrate the problems associated with prior art linearization approaches. Within this assumed operating range voltages close to zero volts are typically representative of thick objects. Since most of the radiation beam is attenuated by a thick object the residual beam I is of low intensity resulting in the detector developing a low voltage. Conversely, voltages close to 10 volts are representative of thin objects. Assume further, again for the sake of illustration, that it is desirable to produce a linear signal having a one mullivolt resolution over the 10 volt range, i.e., a one part in 10,000 resolution. Several types of linearization systems are known which are capable of developing such a high resolution signal. These systems and their shortcomings will be discussed immediately hereinafter.
The first type of known system employs a memory to store non-linear correction coefficients each of which may be used to develop, typically via a D/A converter, one of the 10,000 voltage levels required to obtain a one part in 10,000 resolution. In these first type systems the non-linear analog signal is used merely to address the memory in order to locate the appropriate correction coefficient. Obviously the memory requirements of such systems are very large. To achieve a one part in 10,000 resolution, 10,000 memory locations are required. A standard 16K memory system is typically employed to provide the required 10,000 memory locations.
An example of a linearization system that utilizes a memory addressed by a non-linear signal to develop a linear output signal may be seen in Muehllehner U.S. Pat. No. 3,745,345, issued July 10, 1973.
A second type of known linearization system minimizes memory requirements by employing a relatively powerful computing system to process the non-linear signal. The computing system directly develops a linear signal corresponding to the non-linear input without having to store all correction coefficient possibilities. Such a system is taught by Tsujii et al. in U.S. Pat. No. 3,955,086, issued May 4, 1976. In particular, Tsujii et al. in FIG. 2 shows "operator" 30 which is a computing system used in conjunction with a D/A converter to directly convert a non-linear signal to a linear signal.
In light of factors such as cost, physical housing constraints, power requirements, etc., it is clearly desirable to be able to develop a high resolution linear analog signal from a non-linear analog signal without requiring either a large memory system or powerful computing device.
A third type of known linearization system operates according to what is hereinafter referred to as a "Standardization" approach. This approach is taught in the Tsujii et al. patent referred to above and is further exemplified by Cho et al. in U.S. Pat. No. 3,729,632, issued Apr. 24, 1973.
According to one Standardization approach an object of known thickness is first inserted into the gauge to develop a first signal representing the standard thickness. This first signal (or digital representation thereof) is compared at some point in time with a second signal (or digital representation thereof) developed when an object of unknown thickness is inserted into the gauge.
The measure of deviation of the second signal from the first signal is indicative of the unknown thickness. As the thickness of the non-standard object increasingly deviates from the sample thickness the probability increases that error is developed in the resulting deviation derived thickness measurement signal. These thickness measurement errors are compounded when linearization is performed. Thus, thickness representative signals generated by a Standardization approach are typically inaccurate.
Finally, a problem common to all known linearization systems is developing output signals which accurately represent object thickness in the noisy environment of a radiation gauge. Statistical fluctuations in the amount of radiation emitted by the radiation source typically appear as variations in gauge output thereby producing gauge output readings which may vary for even the same object.
In light of all of the problems stated hereinabove, it is an object of the invention to develop a high resolution linear analog output signal from a high resolution non-linear analog input signal while requiring only a minimal memory configuration and a minimal amount of computing power.
It is still a further object of the invention to develop a high resolution linear analog output signal representative of the unknown thickness of an object in a "dynamic" manner, i.e., without reference to a standard, to thereby eliminate the aforementioned significant errors typically generated when using a Standarization approach.
Further yet, it is an object of the invention to compensate for the statistical fluctuations in the amount of radiation emitted by the radiation source to thereby yield very accurate gauge output readings.